1. Field of the Invention
The present invention relates to effective technique for an object detector using a scanning antenna.
2. Description of the Prior Art
Technique for detecting an object by scanning beams in a given region with a scanning antenna and analyzing an echo reflected by an object existing in the detection region is well known. Such remote sensing technique using the electric wave is applied to an electric wave sensor such as a detector of an intruder for security or a car detector for a vehicle, for example.
In the case of the electric wave sensor using the scanning antenna, since it is necessary to distinguish between a detection object and another object and specify an accurate bearing of the detection object, superior directional characteristics are demanded as antenna characteristics in general. In addition, in order to improve bearing resolution, it is desirable that the irradiation direction of the beam can be varied finely.
However, when the directional characteristics of the antenna are enhanced to narrow an angle of a mainlobe or to increase a gain, a level of a sidelobe is raised (performance of sidelobe suppression is lowered). In this case, there arises a problem that an object existing in unintended bearing is detected because of the sidelobe and the bearing of the detection object could be misjudged (directional error detection).
Especially, in the case of the electronic scanning antenna which electronically varies the directional characteristics, since a larger sidelobe is generated as compared with a mechanical scanning antenna, the directional error detection due to the sidelobe becomes a serious problem.
In order to solve the above problem, various attempts have been made to reduce an influence of the sidelobe. For example, a method of removing a sidelobe pattern from level distribution by beat frequency.
Another example is a method of suppressing the sidelobe by devising constitutions of a transmission antenna and a reception antenna.
However, since only the mainlobe contributes to the object detection in either method, it is necessary to enhance the directional characteristics of the antenna and to control the variation amount in the mainlobe irradiation direction finely in order to improve the bearing resolution.
Therefore, in the case of the electronic scanning type of the phased array antenna, the number of the antenna elements has to be increased and performance of a phase shifter has to be enhanced, which causes the constitution to be complicated and enlarged and increasing the cost. Therefore, its practical application is difficult at a consumer level. In addition, in the case of the mechanical scanning type of the antenna also, since signal processing speed has to be increased due to the increase in the number of sampling of the echo and an antenna angle has to be controlled with high precision, its practical application is difficult also from a view point of the cost.
Besides, in order to enhance the directional characteristics of the antenna and improve the performance of the sidelobe suppression, designing and manufacturing of the antenna and a line length of its electric circuit becomes sever, which increases the cost also.